Methods for forming layers within a MEMS device using liftoff processes to achieve a tapered edge

ABSTRACT

Certain MEMS devices include layers patterned to have tapered edges. One method for forming layers having tapered edges includes the use of an etch leading layer. Another method for forming layers having tapered edges includes the deposition of a layer in which the upper portion is etchable at a faster rate than the lower portion. Another method for forming layers having tapered edges includes the use of multiple iterative etches. Another method for forming layers having tapered edges includes the use of a liftoff mask layer having an aperture including a negative angle, such that a layer can be deposited over the liftoff mask layer and the mask layer removed, leaving a structure having tapered edges.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 60/710,019, filed Aug. 19, 2005, whichis hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and/or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF THE INVENTION

In one embodiment, a method of fabricating a MEMS device is provided,the method including forming an electrode layer over a substrate,forming a sacrificial layer over the electrode layer, patterning thesacrificial layer to form at least one tapered aperture extendingthrough the sacrificial layer, forming a mask layer including anaperture overlying the tapered aperture extending through thesacrificial layer, the mask layer including an overhanging portionextending around the aperture in the mask layer, depositing a supportlayer over the mask layer, removing the mask layer via a liftoffprocess, thereby forming a support structure located at least partiallywithin the tapered aperture extending through the sacrificial layer, andforming a movable layer adjacent the support structure.

In another embodiment, a method of fabricating a MEMS device isprovided, the method including forming an electrode layer over asubstrate, forming a mask over the electrode layer, where the maskincluding a negative angle extending about the periphery of the mask,depositing a layer of sacrificial material over the mask, removing themask via a liftoff process, forming a sacrificial layer including atapered aperture, forming a support structure located at least partiallywithin the tapered aperture, and forming a movable layer adjacent thesupport structure.

In another embodiment, a method of fabricating a MEMS device isprovided, the method including forming a lower electrode layer over asubstrate, forming a sacrificial layer over the electrode layer, forminga mask layer overlying the sacrificial layer, the mask layer includingan aperture having an overhanging portion extending around an edge ofthe aperture, depositing a layer of electrode material over the masklayer, removing the mask layer via a liftoff process, thereby forming atleast an isolated electrode member having an outwardly tapering edge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a top plan view of an array of interferometric modulatorelements in which the individual elements comprise support structures.

FIGS. 9A-9J are schematic cross-sections illustrating a method forfabricating an interferometric modulator element comprising supportstructures located over a movable layer.

FIGS. 10A-10F are schematic cross-sections illustrating a method forfabricating an interferometric modulator element comprising a reflectivelayer which is partially separated from a mechanical layer.

FIGS. 11A-11B are schematic cross-sections illustrating a method forforming a MEMS structure having a tapered edge through the use of anetch leading layer.

FIGS. 12A-12B are schematic cross-sections illustrating an alternatemethod for forming a MEMS structure having a tapered edge through theuse of an etch leading layer.

FIG. 13 is a schematic cross-section illustrating a step in a method forforming a MEMS structure having a tapered edge by altering theproperties of the support structure layer during formation of the MEMSstructure layer.

FIG. 14 is a schematic cross-section illustrating a step in a method forforming a MEMS structure having a tapered edge by forming a series ofsublayers having differing properties to form the MEMS structure layer.

FIG. 15 is a schematic cross-section illustrating a step in a method forforming a MEMS structure having a tapered edge by forming an overlyingmask layer having poor adhesion to the MEMS structure layer.

FIGS. 16A-D are schematic cross-sections illustrating a method forforming a MEMS structure having a tapered edge through the use of aniterative ashing and etching process.

FIGS. 17A-17E are schematic cross-sections illustrating a method forforming a MEMS structure having a tapered edge through the use ofmultiple etches.

FIGS. 18A-18C are schematic cross-sections illustrating a method forforming a support structure having tapered edges through the use of aliftoff process.

FIGS. 19A-19C are schematic cross-sections illustrating a method forforming a mask layer having a negative taper for use in a liftoffprocess.

FIG. 20 is a schematic cross-section illustrating a step in a method forforming a sacrificial layer having a tapered aperture.

FIG. 21 is a schematic cross-section illustrating a step in a method forforming an isolated electrode member having a tapered edge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

In the fabrication of MEMS devices which comprise a series ofsequentially deposited layers, it is often advantageous to providetapered or angled structures, such as an underlying layer or structurehaving a tapered edge in order to facilitate the conformal deposition ofoverlying layers without breaks or weak thinner portions. In certainembodiments, etching methods can be used to provide such tapered edgesto, e.g., mirror/electrodes or support structures. In particularembodiments, these etching methods may include the use of etch leadinglayers overlying the layers to be etched. In other particularembodiments, these etching methods may include an iterative etchingand/or ashing process. In other embodiments, liftoff processes can beused to form structures having tapered edges.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent, and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. Thus, there exists awindow of applied voltage, about 3 to 7 V in the example illustrated inFIG. 3, within which the device is stable in either the relaxed oractuated state. This is referred to herein as the “hysteresis window” or“stability window.” For a display array having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, pixels in the strobed row thatare to be actuated are exposed to a voltage difference of about 10volts, and pixels that are to be relaxed are exposed to a voltagedifference of close to zero volts. After the strobe, the pixels areexposed to a steady state voltage difference of about 5 volts such thatthey remain in whatever state the row strobe put them in. After beingwritten, each pixel sees a potential difference within the “stabilitywindow” of 3-7 volts in this example. This feature makes the pixeldesign illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or relaxed pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orrelaxed state, is essentially a capacitor formed by the fixed and movingreflective layers, this stable state can be held at a voltage within thehysteresis window with almost no power dissipation. Essentially nocurrent flows into the pixel if the applied potential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bias), and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts, respectively Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +ΔV, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias), or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bias), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to −V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding and vacuum forming. In addition, the housing 41 may be made fromany of a variety of materials, including, but not limited to, plastic,metal, glass, rubber, and ceramic, or a combination thereof. In oneembodiment, the housing 41 includes removable portions (not shown) thatmay be interchanged with other removable portions of different color, orcontaining different logos, pictures, or symbols.

The display 30 of the exemplary display device 40 may be any of avariety of displays, including a bi-stable display, as described herein.In other embodiments, the display 30 includes a flat-panel display, suchas plasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of the exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43, which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment, the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS, or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, the network interface27 can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe memory device such as a digital video disc (DVD) or a hard-disc drivethat contains image data, or a software module that generates imagedata.

The processor 21 generally controls the overall operation of theexemplary display device 40. The processor 21 receives data, such ascompressed image data from the network interface 27 or an image source,and processes the data into raw image data or into a format that isreadily processed into raw image data. The processor 21 then sends theprocessed data to the driver controller 29 or to the frame buffer 28 forstorage. Raw data typically refers to the information that identifiesthe image characteristics at each location within an image. For example,such image characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40. Theconditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. The conditioning hardware 52 may be discretecomponents within the exemplary display device 40, or may beincorporated within the processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, the array driver 22, andthe display array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, the driver controller29 is a conventional display controller or a bi-stable displaycontroller (e.g., an interferometric modulator controller). In anotherembodiment, the array driver 22 is a conventional driver or a bi-stabledisplay driver (e.g., an interferometric modulator display). In oneembodiment, a driver controller 29 is integrated with the array driver22. Such an embodiment is common in highly integrated systems such ascellular phones, watches, and other small area displays. In yet anotherembodiment, the display array 30 is a typical display array or abi-stable display array (e.g., a display including an array ofinterferometric modulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, the input device 48includes a keypad, such as a QWERTY keyboard or a telephone keypad, abutton, a switch, a touch-sensitive screen, or a pressure- orheat-sensitive membrane. In one embodiment, the microphone 46 is aninput device for the exemplary display device 40. When the microphone 46is used to input data to the device, voice commands may be provided by auser for controlling operations of the exemplary display device 40.

The power supply 50 can include a variety of energy storage devices asare well known in the art. For example, in one embodiment, the powersupply 50 is a rechargeable battery, such as a nickel-cadmium battery ora lithium ion battery. In another embodiment, the power supply 50 is arenewable energy source, a capacitor, or a solar cell including aplastic solar cell, and solar-cell paint. In another embodiment, thepower supply 50 is configured to receive power from a wall outlet.

In some embodiments, control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some embodiments, controlprogrammability resides in the array driver 22. Those of skill in theart will recognize that the above-described optimizations may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports 18 at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as supportstructures, which can take the form of isolated pillars or posts and/orcontinuous walls or rails. The embodiment illustrated in FIG. 7D hassupport structures 18 that include support plugs 42 upon which thedeformable layer 34 rests. The movable reflective layer 14 remainssuspended over the cavity, as in FIGS. 7A-7C, but the deformable layer34 does not form the support posts by filling holes between thedeformable layer 34 and the optical stack 16. Rather, the support posts18 are formed of a planarization material, which is used to form thesupport post plugs 42. The embodiment illustrated in FIG. 7E is based onthe embodiment shown in FIG. 7D, but may also be adapted to work withany of the embodiments illustrated in FIGS. 7A-7C, as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

In certain embodiments, it may be desirable to provide additionalsupport to a movable layer such as the movable reflective layer 14illustrated in FIG. 7A, or the combination of mechanical layer 34 andmovable reflective layer 14 of FIGS. 7C-7E. In optical MEMS devices,such as interferometric modulators, the movable layer may comprise areflective sublayer and a mechanical sublayer, as will be discussed ingreater detail below. Such support may be provided by a series ofsupport structures which may be located along the edges of an individualmodulator element and/or in the interior of such an element. In variousembodiments, these support structures may be located either over orunderneath a movable layer. In alternate embodiments, support structuresmay extend through an aperture formed in the mechanical layer, such thatsupport is provided from both above and below the mechanical layer. Asused herein, the term “rivet” generally refers to a patterned layeroverlying a mechanical layer in a MEMS device, usually in a recess ordepression in the post or support region, to lend mechanical support forthe mechanical layer. Preferably, though not always, the rivet includeswings overlying an upper surface of the mechanical layer to addstability and predictability to the mechanical layer's movement.Similarly, support structures underlying a mechanical layer in a MEMSdevice to lend mechanical support for the mechanical layer are generallyreferred to herein as support “posts.” In many of the embodimentsherein, the preferred materials are inorganic for stability relative toorganic resist materials.

An exemplary layout of such support structures is shown in FIG. 8, whichdepicts an array of MEMS elements. In certain embodiments, the array maycomprise an array of interferometric modulators, but in alternateembodiments, the MEMS elements may comprise any MEMS device having amovable layer. It can be seen that support structures 62, which in theillustrated embodiment are underlying post structures but may in otherembodiments include overlying rivet structures, are located both alongthe edges of a movable layer 66 and in the interior of a MEMS element,in this example an interferometric modulator element 60. Certain supportstructures may comprise rail structures 64, which extend across the gap65 between two adjacent movable layers 66. It can be seen that movablelayer 66 comprises a strip of deformable material extending throughmultiple adjacent elements 60 within the same column. The railstructures 64 run parallel with lower electrodes which define rowscrossing the upper electrodes defined by the strips of the movable layer66. The support structures 62 serve to stiffen the movable layer 66within the elements or pixels 60, and together with the rail structures64, separate the upper and lower electrodes to define cavities in whichthe upper electrode can move vertically.

Advantageously, these support structures 62 are made small relative tothe surrounding area of the modulator element 60. As the support postsconstrain deflection of the movable layer 66 and may generally beopaque, the area underneath and immediately surrounding the supportstructures 62 is not usable as active area in a display, as the movablelayer in those areas is not movable to a fully actuated position (e.g.,one in which a portion of the lower surface of the movable layer 14 ofFIG. 7A is in contact with the upper surface of the optical stack 16).Because this may result in undesirable optical effects in the areassurrounding the post, a dark or “black” mask layer may advantageously beprovided between the support structures and the viewer to avoidexcessive reflection in these regions that may wash out the intendedcolor.

In certain embodiments, these support structures may comprise adepression in the movable layer, along with a substantially rigidstructure(s) above and/or below the movable layer which helps tomaintain the shape. While such support structures may be formed of apolymer material, an inorganic material having greater rigidity ispreferably used, and provides advantages over similar structurescomprising polymeric materials.

For instance, a polymeric support structure may not maintain a desiredlevel of rigidity over a wide range of operating temperatures, and maybe subject to gradual deformation or mechanical failure over thelifetime of a device. As such failures may affect the distance betweenthe movable layer and the optical stack, and this distance at leastpartially determines the wavelengths reflected by the interferometricmodulator element, such failures may lead to a shift in the reflectedcolor due to wear over time or variance in operating temperatures. OtherMEMS devices experience analogous degradation over time when supportsare formed of polymeric material.

One process for forming an interferometric modulator element comprisingunderlying post support structures is described with respect to FIGS.9A-9J. In FIG. 9A, it can be seen that a transparent orlight-transmissive substrate 70 is provided, which may comprise, forexample, glass or a transparent polymeric material. A conductive layer72, which may comprise indium-tin-oxide (ITO), is then deposited overthe transparent substrate and a partially reflective layer 74, which maycomprise chromium, is deposited over the conductive layer 72. Althoughin one embodiment conductive layer 72 may comprise ITO, and may bereferred to as such at various points in the below specification, itwill be understood that the conductive layer 72 may comprise anysuitable conductive material, and need not be transparent fornon-optical MEMS structures. Similarly, although sometimes referred toas a chromium layer, partially reflective layer 74 may comprise anysuitable partially reflective layer, and may be omitted for non-opticalMEMS structures.

The conductive layer 72 and partially reflective layer 74 are thenpatterned and etched to form bottom electrodes, also referred to as rowelectrodes, which run cross-wise (e.g., perpendicular) to the movablelayer 66 of FIG. 8 and which will be used to address a row of MEMSelements. In certain embodiments, the conductive and partiallyreflective layers 72 and 74 may advantageously also be patterned andetched to remove the ITO and chromium underlying the areas where thesupport post structures will be located, forming apertures 76 asdepicted in FIG. 9B. This patterning and etching is preferably done bythe same process which forms the row electrodes. The removal of ITO andchromium (or other conductive materials) underlying the supportstructures helps to minimize risk of shorting between an overlyingconductive layer, such as the movable layer, and the bottom electrode.Thus, FIG. 9B and the subsequent figures depict a cross-section of acontinuous row electrode formed by layers 72 and 74, in which isolatedapertures 76 have been etched, taken along a line extending throughthose apertures. In other embodiments in which the conductive layer 72and partially reflective layer 74 are not etched to form apertures 76, adielectric layer, discussed below, may provide sufficient protectionagainst shorting between the bottom electrode and the movable layer.

The conductive layer 72 and partially reflective layer 74 may bepatterned via photolithography and etched via, for example, commerciallyavailable wet etches. Chromium wet etches include solutions of aceticacid (C₂H₄O₂) and cerium ammonium nitrate [Ce(NH₄)₂(NO₃)₆]. ITO wetetches include HCl, a mixture of HCl and HNO₃, or a mixture ofFeCl₃/HCl/DI in a 75%/3%/22% ratio and H₂O. Once the apertures 76 havebeen formed, a dielectric layer 78 is deposited over the conductive andpartially reflective layers 72 and 74, as seen in FIG. 9C, forming theoptical stack 16. In certain embodiments, the dielectric layer maycomprise SiO₂ or SiN_(x), although a wide variety of suitable materialsmay be used.

The thickness and positioning of the layers forming the optical stack 16determines the color reflected by the interferometric modulator elementwhen the element is actuated (collapsed), bringing the movable layer 66into contact with the optical stack. In certain embodiments, the opticalstack is configured such that the interferometric modulator elementreflects substantially no visible light (appears black) when the movablelayer is in an actuated position. Typically, the thickness of thedielectric layer 78 is about 450 Å, although it will be understood thatthe desired thickness will vary based on both the refractive index ofthe material and the desired color reflected by the interferometricmodulator in a collapsed state. While illustrated for simplicity asplanar (which can be achieved if the dielectric layer 78 is a spin-onglass), the dielectric layer 78 is typically conformal over thepatterned lower electrode formed from layers 72 and 74.

As seen in FIG. 9D, a layer 82 of sacrificial material is then depositedover the dielectric layer 78. In certain embodiments, this sacrificiallayer 82 is formed from a material which is etchable by fluorine-basedetchants, particularly XeF₂. For example, the sacrificial layer 82 maybe formed from molybdenum or amorphous silicon (a-Si). In otherembodiments, the sacrificial layer may comprise tantalum or tungsten.Other materials which are usable as sacrificial materials includesilicon nitride, certain oxides, and organic materials. The thickness ofthe deposited sacrificial layer 82 will determine the distance betweenthe optical stack 16 and the movable layer 66, thus defining thedimensions of the interferometric gap 19 (see FIG. 7A). As the height ofthe gap 19 determines the color reflected by the interferometricmodulator element when in an unactuated position, the thickness of thesacrificial layer 82 will vary depending on the desired characteristicsof the interferometric modulator. For instance, in an embodiment inwhich a modulator element that reflects green in the unactuated positionis formed, the thickness of the sacrificial layer 82 may be roughly 2000Å. In further embodiments, the sacrificial layer may have multiplethicknesses across an array of MEMS devices, such as in a multicolordisplay system where different interferometric gap sizes are used toproduce different colors.

In FIG. 9E, it can be seen that the sacrificial layer 82 has beenpatterned and etched to form tapered apertures 86. The apertures 86overlie the apertures 76 cut into the layers 72 and 74 of ITO andchromium. These apertures 86 may be formed by masking the sacrificiallayer, using photolithography, and then performing either a wet or dryetch to remove portions of the sacrificial material. Suitable dry etchesinclude, but are not limited to, SF₆, CF₄, Cl₂, or any mixture of thesegases with O₂ or a noble gas such as He or Ar. Wet etches suitable foretching Mo include a PAN etch, which may be a mix of phosphoric acid,acetic acid, nitric acid and deionized water in a 16:1:1:2 ratio.Amorphous silicon can be etched by wet etches including KOH and HFNitrate. Preferably, however a dry etch is used to etch the sacrificiallayer 82, as dry etches permit more control over the shape of taperedapertures 86.

In FIG. 9F a layer 84 of inorganic post material is deposited over thepatterned sacrificial layer 82, such that the inorganic post layer 84also coats the side walls and the base of the tapered apertures 86. Incertain embodiments, the inorganic post layer 84 is thinner than thesacrificial layer 82, and is conformal over the sacrificial layer 82. Incertain embodiments, the inorganic post layer 84 may comprise siliconnitride (SiN_(x)) or SiO₂, although a wide variety of other materialsmay be used, some of which are discussed in greater detail below.

In FIG. 9G, the inorganic post layer 84 is patterned and etched to forminorganic posts 88. Thus, the inorganic post layer 84 is preferablyselectively etchable with respect to the underlying sacrificial layer82, so as to permit etching of inorganic post layer 84 while leaving thesacrificial layer 82 unaffected. However, if the inorganic post layer 84is not selectively etchable relative to the sacrificial layer 82, anetch stop layer (not shown) may be provided between the inorganic postlayer 84 and the sacrificial layer 82.

It can be seen in FIG. 9G that the edges of the inorganic posts 88preferably taper which, like the tapered or sloped sidewalls of theapertures 86, facilitates continuous and conformal deposition ofoverlying layers. It can be seen that the post structure 88 in theillustrated embodiment has a thickness which is thinner than that of thesacrificial layer 82, and comprises a substantially flat base portion89, a sloped sidewall portion 87, and a substantially horizontal wingportion 85 which extends over a portion of the sacrificial material.Thus, the post 88 advantageously provides a substantially flat surfaceat the edge of the post for supporting an overlying movable layer 66(see FIG. 9H), minimizing stress and the resultant undesired deflectionwhich might occur if the movable layer 66 were deposited over a lessflat edge. Details of how to taper structures like the post 88 of FIG.9H are discussed with respect to FIGS. 11A-21, below.

In one embodiment, the inorganic post layer 84 and resultant post 88comprise diamond-like carbon (DLC). In addition to being extremely hardand stiff (roughly 10× harder than SiO₂), the DLC inorganic post layer84 can be etched with an O₂ dry etch. Advantageously, an O₂ dry etch ishighly selective relative to a wide variety of sacrificial materials,including but not limited to Mo and a-Si sacrificial material, as wellas other sacrificial materials discussed above. An inorganic postcomprising DLC thus provides a very stiff post, lessening the likelihoodand amount of downward flexure of the edges of the support post 88 whenoverlying moving or mechanical layers are pulled downward during MEMSoperation, while permitting the use of an etch which is relativelybenign to a wide variety of materials.

In FIG. 9H, it can be seen that the components which will form themovable layer 66 (see, e.g., moveable reflective layer 14 in FIG. 7A)are then deposited over the etched sacrificial layer 82, lining thetapered apertures 86. In the embodiment of FIG. 9H, a highly reflectivelayer 90, also referred to as a mirror or mirror layer, is depositedfirst, followed by a mechanical layer 92. The highly reflective layer 90may be formed from a specular metal, such as aluminum or an aluminumalloy, due to their high reflectance over a wide spectrum ofwavelengths. The mechanical layer 92 may comprise a metal such as Ni andCr, and is preferably formed such that the mechanical layer 92 containsresidual tensile stress. The residual tensile stress provides themechanical force which tend to pull the movable layer 66 away from theoptical stack 16 when the modulator is unactuated, or “relaxed.” Forconvenience, the combination of the highly reflective layer 90 andmechanical layer 92 is collectively referred to as the movable layer 66,although it will be understood that the term movable layer, as usedherein, also encompasses a partially separated mechanical and reflectivelayer, such as the mechanical layer 34 and the movable reflective layer14 of FIG. 7C.

In an embodiment in which the sacrificial layer is to be “release”etched by a XeF₂ etch, both the reflective layer 90 and the mechanicallayer 92 are preferably resistant to XeF₂ etching. If either of theselayers is not resistant, an etch stop layer may be used to protect thenon-resistant layer surface exposed to the release etch. Similarly, thepost 88 is preferably resistant to the release etch, or is alternatelyprotected by an etch stop layer. It can also be seen that the taper ofthe edges of the posts 88 facilitates the conformal deposition of thereflective layer 90 and mechanical layer 92. Absent this taper, it maybe difficult to deposit these layers such that the layers havesubstantially uniform thicknesses over surfaces outside and within theapertures 86.

In an alternate embodiment, the movable layer 66 may be a single layerwhich is both highly reflective and has the desired mechanicalcharacteristics. However, the deposition of two distinct layers permitsthe selection of a highly reflective material, which might otherwise beunsuitable if used as the sole material in a movable layer 66, andsimilarly allows selection of a suitable mechanical layer (with someflexibility and inherent tension) without regard to its reflectiveproperties. In yet further embodiments, the movable layer may be areflective sublayer which is largely detached from the electrical andmechanical layer, such that the reflective layer may be translatedvertically without bending (see, e.g., FIGS. 10A-10F and attendantdescription).

In other embodiments in which the MEMS devices being formed comprisenon-optical MEMS devices (e.g., a MEMS switch), it will be understoodthat the movable layer 66 need not comprise a reflective material. Forinstance, in embodiments in which MEMS devices such as MEMS switches arebeing formed comprising the support structures discussed herein, theunderside of the movable layer 66 need not be reflective, and mayadvantageously be a single layer, selected solely on the basis of itsmechanical properties or other desirable properties.

Next, in FIG. 9I, it can be seen that photolithography is used topattern the mechanical layer 92, and etch the movable layer 66 (i.e.,the mechanical layer 92 and the reflective layer 90) to form etch holes100, which expose portions of the sacrificial layer 82, in order tofacilitate “release” etching of the sacrificial layer. In certainembodiments, multiple etches are employed to expose the sacrificiallayer. For example, if the mechanical layer 92 comprises nickel and thereflective layer 90 comprises aluminum, HNO₃ may be used to etch themechanical layer 92, and phosphoric acid or a base such as NH₄OH, KOH,THAM, or NaOH may be used to etch the reflective layer 90. Thispatterning and etching may also be used to define the strip electrodesseen in FIG. 8, by etching gaps 65 between strips of the movable layer66 (see FIG. 8), separating columns of MEMS devices from one another.

Finally, in FIG. 9J, it can be seen that a release etch is performed toremove the sacrificial layer, creating the interferometric gap 19through which the movable layer 66 can move. In certain embodiments, aXeF₂ etch is used to remove the sacrificial layer 82. Because XeF₂etches the preferred sacrificial materials well, and is extremelyselective relative to other materials used in the processes discussedabove, the use of a XeF₂ etch advantageously permits the removal of thesacrificial material with very little effect on the surroundingstructures.

As discussed above, certain embodiments of MEMS devices, and inparticular interferometric modulators, comprise a movable layercomprising a reflective layer which is partially detached from amechanical layer. FIGS. 10A-10F illustrate an exemplary process forforming separate mirror structures underlying the mechanical layer insuch a MEMS device, which in the illustrated embodiment is aninterferometric modulator. This process may include, for example, thesteps described with respect to FIGS. 9A-9D, in which an optical stackis deposited, and a sacrificial layer is deposited over the opticalstack.

In FIG. 10A, it can be seen that a reflective layer 90 is deposited overthe sacrificial layer 82. In certain embodiments, the reflective layer90 may comprise a single layer of reflective material. In otherembodiments, the reflective layer 90 may comprise a thin layer ofreflective material with a layer of more rigid material (not shown)overlying the thin layer of sacrificial material. As the reflectivelayer of this embodiment will be partially detached from an overlyingmechanical layer, the reflective layer 90 preferably has sufficientrigidity to remain in a substantially flat position relative to theoptical stack 16 even when partially detached, and the inclusion of astiffening layer on the side of the reflective layer located away fromthe optical stack can be used to provide the desired rigidity.

In FIG. 10B, the reflective layer 90 of FIG. 10A is patterned to form apatterned mirror layer 200. In one embodiment, the patterned mirrorlayer 200 comprises a contiguous layer in which apertures correspondingto the locations of (but wider or narrower than) support structures havebeen formed. In another embodiment, the patterned mirror layer 200 maycomprise multiple reflective sections detached from one another.

In FIG. 10C, a second sacrificial layer 196 is deposited over thepatterned mirror layer 200. Preferably, the second sacrificial layer 196is formed from the same material as the first sacrificial layer 82, oris etchable selectively with respect to surrounding materials by thesame etch as the first sacrificial layer 82.

In FIG. 10D, tapered apertures 86 are formed which extend through boththe second sacrificial layer 196 and the first sacrificial layer 82. Itcan also be seen in FIG. 35D that an aperture 208 is formed in a portionof the second sacrificial layer 196 overlying the patterned mirror layer200, exposing at least a portion of the patterned mirror layer 200.

In FIG. 10E, a mechanical layer 92 is deposited over the patternedsacrificial layers 196 and 82 and exposed portions of the patternedmirror layer 200. In particular, it can be seen that the mechanicallayer 92 at least partially fills the aperture 208 (see FIG. 10D), suchthat a connector portion 202 connecting the mechanical layer 92 and thepatterned mirror layer 200 is formed.

In FIG. 10F, a release etch is performed which removes both the firstsacrificial layer 82 and the second sacrificial layer 196, therebyforming an interferometric gap 19 between the patterned mirror layer 200and the optical stack. Thus, an optical MEMS device the formed, whichincludes a movable layer 66 comprising a mechanical layer 92 from whicha patterned mirror layer 200 is suspended, where the patterned mirrorlayer 200 is partially detached from the mechanical layer 92. Thisoptical MEMS device, may be, for example, an interferometric modulatorsuch as that described with respect to FIG. 7C and elsewhere throughoutthe application. In non-optical MEMS, the suspended upper electrode neednot be reflective.

In the embodiment of FIG. 10F, it is undesirable for the patternedmirror layer 200 to have a re-entrant edge surface, as undercutting themirror layer may lead to undesirable optical effects if incident lightis reflected off of the re-entrant edge surface and reflected towardsthe viewer. Desirably, the patterned mirror layer 200 comprises a flatlower surface, such that any light reflected by the patterned mirrorlayer 200 is reflected in a uniform direction. One method for ensuringthat the patterned mirror layer 200 comprises a substantially flatsurface without significant undercut, is to etch the mirror layer suchthat the patterned mirror layer comprises a tapered edge. In addition,if the patterned mirror layer 200 comprises a tapered edge, the secondsacrificial layer and other overlying layer s9 e.g., the mechanicallayer) can be more reliably deposited conformally over the patternedmirror layer 200, with less risk of thickness nonuniformity and stressescreated at sharper 90 degree corners.

Thus, it is often desirable to form tapered edges in a variety oflocations throughout a MEMS device, including when overlying layers areto be deposited conformally over the patterned layer, or when it isdesirable to avoid a re-entrant profile. Several methods are disclosedbelow for the formation of such tapered edges. While they are describedprimarily with respect to the post embodiment discussed above withrespect to FIGS. 9A-9J, other uses of the structures and methods arecontemplated (such as in the patterning of the mirror layer 200 of FIGS.10A-10F) and can be achieved through modification of the belowstructures and methods. In certain embodiments, these processes includepatterning a layer to form a desired structure and tapering the edge ofthat structure. Although sometimes described as two distinct steps, itwill be understood that the patterning and tapering will often comprisea single step, and that in further embodiments, partial tapering willoccur while the structure is being patterned, such that the patterningand tapering need may be done simultaneously.

In one embodiment, described with respect to FIGS. 11A-11B, an etchleading layer is used to achieve an etch having the desired taper. InFIG. 11A, it can be seen that a layer of inorganic post material 84 isdeposited over a sacrificial layer 82. An etch leading layer 270 isdeposited over the post layer 84, and a mask 272 is formed over the areawhere the inorganic post will be. In this embodiment, the etch leadinglayer is selected to be a material which, for particular etches, willhave an etch rate which is faster than the etch rate of the underlyingmaterial, or the post material 84 in the illustrated embodiment. Incertain embodiments, the post layer 84 may comprise SiO₂ or SiN_(x). Theetch leading layer may comprise a wide variety of materials, includingan SiO₂ or SiN_(x) layer which are deposited at a low temperature; aresignificantly more porous than the underlying layer, are deposited witha different composition of nitrogen in the SiN_(x) layer; or are formedto be particularly oxygen or nitrogen rich. Similarly, a hydrogen richSiN_(x) layer may be used, or a layer of SiN_(x) deposited at lowtemperatures. Si is etched faster than SiO₂ when a HF/HNO₃ etch is used,and Al will etch faster than SiO₂ when an HF+H₃PO₄ etch is used. Also,Mo will etch faster than SiO₂ when either a HF/HNO₃ or an HF/HNO₃/H₃PO₄etch is used. BHF is commonly used to etch SiN_(x), although othersuitable etchants may be used. A wide variety of other etches and etchleading layers may be used depending on the composition of the inorganicpost layer 84. Depending on the etchant to be used, the mask 272 may beformed of, for example, a hard mask such as Ni, or a photoresist mask.In particular, when a wet etch is used, the mask 272 may comprise a hardmask.

FIG. 11B depicts the embodiment of FIG. 11A after being exposed to anetchant, preferably an isotropic etchant. As can be seen, the etchleading layer 270 has been etched faster than the post layer 84,exposing both the top and the side of the post layer 84 to the etchant.Because the post layer 84 is etched from the top and from the side, atapered side has been formed by this etch. Subsequently, one or both ofthe mask 272 and the remaining etch leading layer 270 may be removed(not shown).

The structure and method of FIGS. 11A-11B may be adapted for use informing a patterned mirror layer having a tapered edge. In oneembodiment, the inorganic post layer 84 of FIGS. 11A-11B may be replacedby a layer of aluminum, and the etch leading layer 270 may be, forexample, a layer of aluminum deposited such that it will etch faster.This can be achieved, for instance, though deposition of the etchleading layer at a higher pressure than the deposition of the loweraluminum layer. In an alternate embodiment, the etch leading layer maycomprise an aluminum alloy having a higher etch rate than aluminum. Instill other embodiments, any suitable material having the desiredmechanical properties and a higher etch rate than aluminum may be used.

A variation of the above embodiment is discussed with respect to FIGS.12A-12B, and includes the step of FIG. 11A. In this embodiment, the etchleading layer 270 and the underlying layer (in the illustratedembodiment, inorganic post material 84) are selectively etchablerelative to one another. In FIG. 12A, it can be seen that the etchleading layer 270 has been selectively etched relative to the post layer84 via a first (preferably isotropic) etch, exposing part of the top ofthe post layer 84. In FIG. 12B, a second (preferably isotropic) etch hasbeen used to etch the post layer 84 both from above and from the side,resulting in the tapered edge seen in the figure. The angle of the tapercan be controlled both by the extent to which the first etch ispermitted to remove the etch leading layer 270, and by the rate at whichthe second etch diffuses into the space between the mask 272 and thepost layer 84. If the second etch etches the post layer 84 such that itundercuts the etch leading layer 270, a third etch, which may use thesame etch as the first etch, may be performed in order to remove theoverhanging portion of the etch leading layer 270, such that a taperededge is formed without undesirable overhanging portions.

The structure and method of FIGS. 12A-12B may also be adapted for use informing a patterned mirror layer having a tapered edge. In oneembodiment, the inorganic post layer 84 of FIGS. 12A-12B may be replacedby a layer of aluminum, and the etch leading layer 270 may comprise alayer of nickel, which can be selectively etched relative to aluminum.The nickel etch leading layer may first be etched, such as via a HNO₃wet etch, which is selective with respect to the aluminum layer. Thealuminum layer may then be etched via an H₃PO₄ wet etch, which isselective with respect to the nickel layer. If necessary, a subsequentHNO₃ etch is performed to remove any portions of the nickel layeroverhanging the tapered aluminum layer.

As discussed above, one or both of the etch leading layer and the maskmay be removed after etching the inorganic post layer. In a furtherembodiment, the etch leading layer of any of the embodiments discussedherein may be selected so that it can be etched away or otherwiseremoved in a single process. In a particular embodiment, the etchleading layer is a polymeric material, such as PMGI or PMMA, and themask is a photoresist mask. The undercut can be achieved during thephotolithography process and it does not require an extra etch step. Inthis embodiment, a single etch (different from the first etch) an removeboth the PR mask and the polymeric etch leading layer, simplifying thefabrication process.

In another embodiment, the desired taper can be achieved through thedeposition of a post layer 84 which has differing properties atdifferent heights in the layer, such that the upper portion of the layer84 will be etched at a faster rate than the lower portion of the layer84, rather than employing a separate etch leading layer. FIG. 13 depictssuch an embodiment, in which the properties of the layer 84 are variedduring the formation (e.g., deposition) of the layer, such that thelayer 84 has been etched into a tapered shape. Such varying propertiescan be achieved in multiple ways. For example, during plasma chemicalvapor deposition, process conditions such as the gas, power and/orpressure may be varied (graded through the thickness) in order to makethe composition of later-deposited material more quickly etchable by aparticular etch. This variation can also be achieved while using asputter etch process

Similarly, as can be seen in FIG. 14, a single layer 84 of inorganicpost material with varying properties can be approximated through thedeposition of multiple layers having slightly different properties. Inthis embodiment, three separate layers 84 a, 84 b, and 84 c arefabricated, such that layers 84 b and 84 c have an etch rate faster than84 a, and 84 c has an etch rate faster than 84 b, upon exposure to aparticular etchant. In various embodiments, more or less layers ofvarying or uniform thickness can be deposited to form such a stratifiedpost layer. As discussed above, various properties of a CVD or sputteretch process can be modified between the deposition of the various postlayers in order to achieved the desired relative etch rates for eachlayer. This process can also be applied to the deposition of a mirrorlayer or any other layer.

In another embodiment, as depicted in FIG. 15, a mask layer 272 can bedeposited over the post layer 84 such that the adhesion between the mask272 and the post layer 84 is intentionally poor. During etching of thepost layer 84, the mask 272 will pull away from the post layer 84,permitting the post layer 84 to be etched from above, as well as fromthe side, creating the desired tapered shape. Preferably, the etch usedto etch the post layer 84 is a wet etch or an isotropic dry etch. Theresist adhesion can be modified during the fabrication process by, forexample, lowering the bake temperature of the mask 272, or chemicallytreating the upper surface of the post layer 84 to reduce adhesion withthe resist 272 or other mask material. This process can also be appliedto the etching of a mirror layer or any other layer.

In yet another embodiment, a tapered edge can be approximated throughthe deposition of a photoresist mask over the inorganic post layer andthe use of successive etching and ashing processes to gradually removeportions of that photoresist mask, forming a staircase-like structure.FIGS. 16A-16D depict a method of forming such a staircase pattern at theedge of an inorganic structure such as the post of FIG. 9J. In FIG. 16A,it can be seen that an etch barrier layer 280 has been formed between alayer 84 of post material and the sacrificial layer 82. A mask layer 282is then deposited over the post layer 84.

In FIG. 16B, an etch is performed to remove a portion of the exposedpost layer 84. As shown in the illustrated embodiment, this etch mayonly remove a portion of the exposed post layer 84, but in otherembodiments, the first etch may etch the entire exposed portion of thepost layer, stopping on the etch stop layer 280.

In FIG. 16C, a portion of the mask 282 has been removed, such as by anashing process, and another etch is performed to remove another newlyexposed portion of the post layer 84. This process is then repeateduntil the post layer 84 has been etched all the way down to the etchbarrier layer 282 in certain places, as can be seen in FIG. 36D. Thus,inorganic posts 290 may be fabricated, having edges with astaircase-like profile that approximates a tapered edge. While anyappropriate number of successive etching and ashing (or other maskreduction or shrinkage) steps can be used, in one embodiment, three suchiterations provide an acceptable approximation of a tapered edge.

This iterative etching and ashing (or other mask reduction or shrinkage)process can also be applied to the etching of a mirror layer. On oneembodiment, the mirror layer is masked, and a first etch, which may be aH₃PO₄ etch, is used to etch a portion of the exposed mirror layer. Themask is then partially ashed, exposing a previously unetched portion ofthe mirror layer, and the mirror layer is then etched via a second etch,which in certain embodiments may be the same as the first etch, and inother embodiments may be a different etch, such as a TMAH etch, whichmay be more selective with respect to the underlying sacrificial or etchstop layer.

In another embodiment, an alternate iterative etching process may beused to form a patterned mirror layer having a tapered edge. In FIG.17A, it can be seen that a reflective layer 90 has been deposited overthe first sacrificial layer 82, and that an etch leading layer 290 hasbeen deposited over the reflective layer 90, followed by a photoresistmask 292. In the illustrated embodiment, the etch leading layer 290 isetchable by an etch (e.g., fluorine-containing) which will also etch thesacrificial layer 82, and in particular may comprise the same materialas the sacrificial layer, e.g., molybdenum.

In FIG. 17B, the reflective layer 90 has been partially etched, such asthrough the use of a PAN etch, which will etch both aluminum andmolybdenum, although it can be seen that the underlying sacrificiallayer 82 has not been exposed by this etch. In FIG. 17C, the remainderof the exposed reflective layer 90 has been etched using an etch whichis selective with respect to the sacrificial material, such as an H₃PO₄etch. By not exposing the underlying sacrificial material to an etchwhich will significantly etch the sacrificial material, undercut of thereflective layer 90 can be avoided, and the desired tapered shapeobtained.

In FIG. 17D, the mask 292 is been stripped. In FIG. 17E, the etchleading layer 290 has been removed by an etch which is selective withrespect to the reflective layer 90, such as an SF₆/O₂ etch. Although aportion of the underlying sacrificial material 92 may be removed by thisetch, it will not have an effect on the tapered shape of the patternedreflective layer 90, as the etch is selective with respect to thereflective layer 90, and will not etch the reflective layer 90 frombeneath.

It will be understood that although the above methods for forming atapered edge have been discussed primarily with respect to shaping theedges of posts, these methods may be modified and applied to formingtapered edges of other layers in the disclosed MEMS devices, such as anelectrode layer or a sacrificial layer, particularly when openingtapered apertures in support regions. In addition, the processesdisclosed and described herein may be modified to incorporate thesesteps whenever the processes could benefit from the etching of a layersuch that it comprises tapered edges. These processes may also beutilized in conjunction with other methods of forming MEMS devices, suchas the disclosed interferometric modulators.

In conjunction with, or in place of, the forming of tapered edges onlayers, the deposition of overlying layers can also be facilitated byseveral alternate methods of improving step coverage at edges of layers.In certain embodiments, the sputter deposition may be biased, so thatsome or all of the material being deposited is deposited at an angle, soas to provide better coverage at corners, reducing the need for taperingthe steps. In another embodiment, the substrate on which these layersare being deposited may be tilted at an angle to the target structure,and in further embodiments may be rotated during deposition. In anotherembodiment, the profile of the target structure used in sputterdeposition may be optimized so that certain portions are at desiredangles to the substrate onto which the material sputtered. In yetanother embodiment, substrate on which the layer is being deposited isrocked back and forth.

Sputter etching may also be advantageously be utilized in the processesdescribed herein. In addition to roughening and cleaning a surface onwhich another layer is to be deposited, as discussed above, sputteretching may be used in conjunction with the sputter deposition. In oneembodiment, a thicker layer than necessary is deposited, and thensputter etched to the desired thickness, providing a smoother layer(e.g., by rounding corners). In a further embodiment, sputter etchingand deposition can be alternated. A biased sputter may also be used toimprove step coverage on conductive substrates, conductive layers, orsubstrates on conductive supports (e.g., chucks). Other depositionprocesses may also be used, such as atomic layer deposition (ALD)atmospheric pressure chemical vapor deposition (AP-CVD), low pressurechemical vapor deposition (LP-CVP), high-density plasma chemical vapordeposition (HDP-CVD), and in further embodiments these depositionprocesses may be modified to include biasing. Any of these methods maybe used in conjunction with any of the processes described herein toimprove step coverage. Furthermore, in certain embodiments, the use ofthese methods may reduce or eliminate the need to form tapered edges onlayers, or augment the advantages of tapered edges.

In other embodiments, lift-off processes can be utilized to form thedesired layers during fabrication of the interferometric modulator. Inone embodiment, described with respect to FIGS. 18A-18C, a lift-offprocess can be utilized to form a desired support structure adjacent amovable layer, and in particular a support post structure underlying themovable layer. Advantageously, this lift-off process permits thedeposition of layers, such as inorganic posts, having the desired taper,facilitating the deposition of overlying layers. In addition, thisprocess may eliminate the need to selectively etch the post materialrelative to the underlying material. In FIG. 18A, it can be seen that alayer 82 of sacrificial material has been patterned and etched to formtapered apertures 86, and that a mask 222 having a negative angle at theedge is deposited over the sacrificial layer 82, such that the mask 222does not extend to the edge of the aperture 86. In certain embodiments,this mask 222 comprises photoresist material (and in a particularembodiment may be a bilayer photoresist, to facilitate formation of thenegative angle), but may also be a hard mask. In a particularembodiment, the mask 222 may comprise the photoresist used to patternthe sacrificial layer 82.

In FIG. 18B, it can be seen that a layer of post material 84 isdeposited over the mask 222. The negative angle of the mask 222 resultsin the deposition of post material 84 in the shape of the desiredinorganic post 188, having the tapered edges due to shadow effects onthe deposition. Depending on the thickness of the mask 222 and thedegree of the negative angle, the shape of the mask 222 alsoadvantageously results in a gap between the post material 84 above themask 222, and the post material 84 which will form the inorganic post.In this embodiment, the mask 222 is advantageously thicker than thematerials to be deposited, permitting access to the mask 222 by etchantor ashing chemicals. Preferably, the post material is deposited viaphysical vapor deposition (e.g., an evaporation or sputtering process),so as to avoid deposition of material on the underside of the mask 222which would inhibit the liftoff of the undesired portion of materialoverlying the mask 222. However, if such connections are formed, forexample if a CVD process is used to deposit the post layer 84,ultrasonic energy can be used to break any connecting portions, as theywill likely be thin in the shadowed region under the mask overhangrelative to the thickness of the post layer 84 generally.

In FIG. 18C, it can be seen that a liftoff process is used to remove themask 222, removing the post material 84 overlying the mask 222 at thesame time. This liftoff process may comprise an etch to remove the mask222, such as a wet etch, or a gaseous or vaporous etch. The liftoffprocess may also comprise a wrinkle bake and rinsing process to removethose materials which are no longer attached. The fabrication processfurther comprises the subsequent deposition of a movable layer,patterning, and releasing the MEMS device, as discussed above.

A similar liftoff process may also be utilized to form rivet structuresoverlying a movable layer. In particular, the process of FIGS. 18A-18Cmay be modified by depositing a movable layer (such as the movable layer66) over the patterned sacrificial layer prior to depositing the mask. Alayer of the desired rivet material is then deposited over the mask, andthe mask is then removed, lifting off the excess rivet layer, forming arivet. Advantageously, as discussed above, this may eliminate the needto selectively etch the rivet layer relative to an underlying mechanicallayer. Thus, a rivet may be formed from the same material as themechanical layer without the need for an etch barrier layer between thetwo layers.

In one embodiment, the mask 222 having a negative angle can be formedthrough the use of a photoresist mask in conjunction with an underlyinglayer of liftoff material. FIGS. 19A-19C depict a process for formingsuch a mask. In FIG. 19A, it can be seen that a layer of liftoffmaterial 224 has been deposited over an underlying layer 226 (which maybe, for example a sacrificial layer), and a layer of photoresistmaterial 228 has been deposited over the liftoff layer 224. In FIG. 19B,the photoresist material 228 has been patterned and selectively removedto form an aperture 229, which will permit etching of the underlyingliftoff layer 224.

In FIG. 19C, it can be seen that the liftoff layer 224 has beenselectively etched with some degree of isotropy to laterally recessunder the photoresist 228 and to form a cavity 230, with edges having anegative angle or overhang, as discussed above. This negative angle canbe achieved, for example, by selectively overetching the liftoff layer224. In one embodiment, the liftoff layer 224 comprises a polyimiderelease layer. In one embodiment, the liftoff layer 224 may itself bephotopatternable. In one embodiment, the liftoff layer 224 may bebilayer photoresist with undercut formed by development. In oneembodiment, as previously depicted in FIG. 18) the mask 222 may beformed from a single liftoff layer, rather than from a liftoff layer inconjunction with a photoresist mask.

A liftoff process may be utilized to form other desired shapes duringthe fabrication of supports for a MEMS device, such as the illustratedinterferometric modulator. In another embodiment, described with respectto FIG. 20, a liftoff process is utilized to form tapered apertures inthe sacrificial layer 82. In this embodiment, a mask 222 is first formedover the optical stack 16, prior to deposition of the sacrificial layer82. As can be seen, this mask 222 comprises negative angles whichcontrol the shape of the deposited sacrificial layer 82. Still withrespect to FIG. 20, it can be seen that a layer 82 of sacrificialmaterial has been deposited, such that the sacrificial material locatedon the substrate will have a shape with the desired tapered aperturesupon liftoff of the mask 222 and the overlying sacrificial material.

In another embodiment, described with respect to FIG. 21, a liftoffprocess is used to form isolated electrode members, which in anembodiment comprising an optical MEMS may be mirrors. This embodimentincludes the steps of FIG. 9A-9D. In FIG. 21, it can be seen that a mask222 is deposited over the sacrificial layer 82, the mask layer having222 a re-entrant profile along the side of the mask layer 222, and thata layer of reflective material 90 is then deposited over the mask layer222. In a later step, the mask 222 is removed by a liftoff process,along with the overlying portions of the reflective layer 90, formingisolated electrode members, which in this embodiment comprise mirrors.The fabrication process may continue as discussed with respect to FIGS.10C-10F.

It will be understood that various combinations of the above embodimentsare possible. Various other combinations of the methods discussed aboveare contemplated and are within the scope of the invention. In addition,it will be understood that structures formed by any of the methods abovemay be utilized in combination with other methods of forming structureswithin MEMS devices.

It will also be recognized that the order of layers and the materialsforming those layers in the above embodiments are merely exemplary.Moreover, in some embodiments, other layers, not shown, may be depositedand processed to form portions of an MEMS device or to form otherstructures on the substrate. In other embodiments, these layers may beformed using alternative deposition, patterning, and etching materialsand processes, may be deposited in a different order, or composed ofdifferent materials, as would be known to one of skill in the art.

It is also to be recognized that, depending on the embodiment, the actsor events of any methods described herein can be performed in othersequences, may be added, merged, or left out altogether (e.g., not allacts or events are necessary for the practice of the methods), unlessthe text specifically and clearly states otherwise.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device of process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers.

1. A method of fabricating a MEMS device, comprising: forming anelectrode layer over a substrate; forming a sacrificial layer over theelectrode layer; patterning the sacrificial layer to form at least onetapered aperture extending through the sacrificial layer; forming a masklayer comprising an aperture overlying the tapered aperture extendingthrough the sacrificial layer, the mask layer comprising an overhangingportion extending around the aperture in the mask layer; depositing asupport layer over the mask layer; removing the mask layer via a liftoffprocess, thereby forming a support structure located at least partiallywithin the tapered aperture extending through the sacrificial layer; andforming a movable layer adjacent the support structure.
 2. The method ofclaim 1, wherein the support structure comprises a tapered edge.
 3. Themethod of claim 1, wherein the support layer is deposited directly overa sidewall of the tapered aperture extending through the sacrificiallayer.
 4. The method of claim 1, additionally comprising performing arelease etch to remove the sacrificial layer, forming an air gap locatedbetween the electrode layer and the movable layer.
 5. The method ofclaim 1, forming the movable layer comprises: forming a reflectivesublayer over the sacrificial layer; and forming a mechanical sublayerover the reflective sublayer.
 6. The method of claim 1, wherein the MEMSdevice comprises an interferometric modulator.
 7. The method of claim 1,wherein the movable layer is formed after forming the support structure.8. The method of claim 1, wherein the mask layer comprises a photoresistmaterial.
 9. The method of claim 8, wherein the mask layer comprises abilayer photoresist.
 10. The method of claim 8, wherein forming the masklayer comprises: depositing a layer of liftoff material; depositing alayer of photoresist material; patterning the photoresist material toform an upper aperture extending through the photoresist material; andetching the liftoff material underlying the upper aperture extendingthough the photoresist material, wherein the etching comprisesundercutting the layer of photoresist material to form the overhangingportion of the mask layer, thereby forming the aperture extendingthrough the mask layer.
 11. The method of claim 8, wherein patterningthe sacrificial layer to form a tapered aperture additionally comprisesusing the mask layer to pattern the sacrificial layer to form a taperedaperture.
 12. The method of claim 1, wherein the support layer isdeposited via an evaportation or sputtering process.
 13. The method ofclaim 1, additionally comprising exposing the support layer toultrasonic energy prior to removing the mask layer via a liftoffprocess.
 14. A MEMS device formed by the method of claim
 1. 15. A methodof fabricating a MEMS device, comprising: forming an electrode layerover a substrate; forming a mask over the electrode layer, wherein themask comprises a negative angle extending about the periphery of themask; depositing a layer of sacrificial material over the mask; removingthe mask via a liftoff process, forming a sacrificial layer comprising atapered aperture; forming a support structure located at least partiallywithin the tapered aperture; and forming a movable layer adjacent thesupport structure.
 16. The method of claim 15, additionally comprisingperforming a release etch to remove the sacrificial layer, forming anair gap located between the movable layer and the electrode layer. 17.The method of claim 15, wherein forming the support structure comprises:depositing a layer of support material over the patterned sacrificiallayer; and patterning the layer of support material to form a supportstructure located at least partially within the aperture, wherein themovable layer is deposited over the support structure.
 18. The method ofclaim 15, wherein the mask comprises a photoresist material.
 19. Themethod of claim 18, wherein the mask comprises a bilayer photoresist.20. The method of claim 18, wherein forming the mask comprises:depositing a layer of liftoff material; depositing a layer ofphotoresist material; patterning the photoresist material to form anaperture extending through the photoresist material; and etching theliftoff material underlying the aperture extending though thephotoresist material, wherein the etching comprises undercutting thelayer of photoresist material to form an overhanging portion of the masklayer.
 21. The method of claim 15, wherein the layer of sacrificialmaterial is deposited via physical vapor deposition process.
 22. A MEMSdevice formed by the method of claim
 15. 23. A method of fabricating aMEMS device, comprising forming a lower electrode layer over asubstrate; forming a sacrificial layer over the electrode layer; forminga mask layer overlying the sacrificial layer, the mask layer comprisingan aperture having an overhanging portion extending around an edge ofthe aperture; depositing a layer of electrode material over the masklayer; removing the mask layer via a liftoff process, thereby forming atleast an isolated electrode member having an outwardly tapering edge.24. The method of claim 23, additionally comprising: forming a secondsacrificial layer overlying the isolated electrode member; patterningthe second sacrificial layer to form an aperture exposing at least aportion of the isolated electrode member; and depositing a mechanicallayer over the second sacrificial layer.
 25. The method of claim 23,additionally comprising removing the sacrificial layer via a releaseetch, thereby forming an air gap located between the isolated electrodemember and the lower electrode layer.
 26. The method of claim 23,wherein the mask layer comprises a photoresist material.
 27. The methodof claim 23, wherein the layer of electrode material is deposited via anevaportation or sputtering process.
 28. A MEMS device formed by themethod of claim 23.